1. Field of the Invention
The invention relates to an aqueous two-component coating composition based on 2-methylpentane-1,5-diisocyanate (referred to below as MPDI).
2. Discussion of the Related Art
Aqueous coating systems are becoming increasingly important, for reasons of economy and ecology. In chemically crosslinking two-component polyurethane coating materials, compounds which include free isocyanate groups ("polyisocyanates"), are used as hardeners for the binder, which is generally an organic polymer having NCO-reactive groups (polyacrylatepolyol, polyetherpolyol or polyesterpolyol). These coating materials have acquired great importance owing to their outstanding properties, in particular their high weathering, resistance to yellowing and chemical stability, and the excellent balance of hardness and flexibility in the coatings produced therewith. It was impossible, until recently, to use such coatings in the absence of an organic solvent (Farbe+Lack 97, 1991, 201-206). For a long time, the use of water in polyurethane coatings appeared impossible because isocyanate groups are known to react not only, as desired, with the organically bonded NCO-reactive groups of the binder but also, undesirably, with water. That reaction produces urea structures and carbon dioxide is liberated. This is said to lead, firstly, to a reduction in the crosslinking density of the cured coating, and, secondly, to the formation of foam as a result of the gas given off. This, in turn, has disadvantageous consequences for the processability of the systems and for the optical properties of the coatings. Overall, aqueous systems of this kind appeared impractical owing to inadequate pot lives and the not entirely satisfactory properties of the coatings.
The hydrolysis of isocyanate groups can be prevented by blocking (or masking) them. For this purpose, the free polyisocyanates are reacted with blocking agents (secondary or tertiary alcohols; lactams or oximes). The blocked polyisocyanates are stable at room temperature and at moderately elevated temperatures and can be blended with the binder to form aqueous one-component coating compositions. At the curing temperatures of the coatings, the blocking agent is eliminated. The isocyanate groups liberated react with the reactive groups of the binder and, in doing so, crosslink the chains. Blocked isocyanates, although avoiding the problems associated with the hydrolysis of the isocyanate groups, nevertheless require an additional process step in terms of blocking and considerably higher curing temperatures and, moreover, in the course of curing, they release the blocking agent which cannot be discharged immediately into the environment.
As a result, there had always been a desire for an aqueous two-component polyurethane coating system with a hardener containing free isocyanate groups. According to DE 27 08 442 and DE 35 29 249, organic di- and/or polyisocyanates are added to polymer dispersions in order to improve the profile of properties. The dispersed polymers, however, are not the organic polyhydroxy compounds usual in polyurethane chemistry as co-reactants for polyisocyanates but are substances which are inert toward isocyanate groups. The property-improving effect is, therefore, attributable not to a chemical crosslinking but to the envelopment of the dispersed polymer with the polyurea compound formed from the isocyanate and water.
Chemically crosslinking aqueous two-component polyurethane coating compositions with free polyisocyanates as hardeners for the binder were, as far as is apparent, described for the first time in EP 0 358 979. According to that patent, polyhydroxy acrylates, as a binder component, are able to emulsify certain polyisocyanates having free isocyanate groups, which are also known as paint polyisocyanates. The resulting aqueous two-component system cures to form crosslinked films. The paint polyisocyanates are oligomeric derivatives, containing biuret, urethane, uretdione and/or isocyanurate groups, of readily available monomeric or simple diisocyanates, in particular of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and/or bis(isocyanatocyclohexyl)methane (H.sub.12 MDI or just HMDI). The paint polyisocyanates used, in accordance with EP 0 358 979, have a viscosity at 23.degree. C. of up to 1000 mPa.cndot.s and an average NCO functionality of from 2.2 to 5. Like the abovementioned blocked polyisocyanates, the paint polyisocyanates are prepared in a separate process step and, because of their relatively high molecular weight, are diluted to application viscosity with organic solvents or are rendered self-dispersible through the incorporation of ionic or nonionic hydrophilic centers.
Further publications deal primarily with expanding the field of use of emulsifying binder components and with varying--which includes nonionichydrophilic modification by alkoxylation--the polyisocyanate hardener components, primarily with the aim of extending the pot life and of improving the profile of properties of the coatings. Hydrophilically modified polyisocyanates, however, present problems as a hardener component, since the cured coatings still contain hydrophilic groups and so are more or less sensitive to water. Examples of binder variations and/or of the use of hydrophilic modified polyisocyanates can be found in EP 0 469 389, 0 496 205, 0 496 210, 0 542 085, 0 542 105, 0 543 228, 0 562 436, 0 583 728, 0 587 061, 0 603 219, 0 610 450, 0 626 401, 0 639 594, 0 654 051, 0 654 052, 0 654 055, 0 657 483, 0 700 945, 0 707 608, 0 731 119, 0 742 239 and 0 746 578. Patents dealing specifically with the preparation of hydrophilically modified polyisocyanates for use in water-dispersed two-component polyurethane systems include EP 0 206 059, 0 516 277, 0 540 985, 0 548 669, 0 645 410, 0 680 983, 0 703 255, 0 728 785 and 0 754 713.
In the case of two-component coating formulations, a fundamental prerequisite for good performance properties is always a particularly fine and highly homogeneous mixing of the crosslinker component with the binder component. This is especially important in the case of aqueous systems, since owing to the chemical nature of the substances used there is no infinite miscibility and so homogeneous, single-phase systems do not result spontaneously; rather, intensive stirring is necessary at the very least. Because of their low viscosity, polyisocyanates based on HDI have been employed primarily to date as a hardener component for aqueous 2-component polyurethane (PU) systems. Nevertheless, polyisocyanates based on IPDI and HMDI can also be used, although, because of the aggregate state, larger amounts of organic solvents are required for formulation than in the case of HDI polyisocyanates, something which is desired to avoid specifically in the case, of environment-friendly aqueous systems.
As mentioned, 2-component PU systems are known for their high weathering, yellowing and chemical stability. According to WO 93/05090, a further improvement in the resistance of the coatings to environmental influences is obtained, in the case of solvent-containing 2-component PU systems, with the use of a mixture of polyisocyanates of HDI with those of IPDI. Adversely affected, on the other hand, is the resistance to mechanical stresses, such as scratch resistance and washline resistance, for example (I-Lack 61, 1993, 30).
An object of this invention is to find new aqueous two-component polyurethane systems in order to obtain improved optical, mechanical, physicochemical and applications-related properties.
This is important in order for the aqueous coating systems to compete successfully with the extremely high quality of solvent-based coating systems, which has not been possible to replace readily by environment-friendly aqueous coating systems without a drop in quality.
The object has been achieved by the aqueous two-component coating compositions of the present invention.